1. Field of the Invention
This invention relates generally to data access arrangements (DAAs). More particularly, it relates to an architecture for a data access arrangement which provides a flat transmit signal into a complex load over all frequencies of interest.
2. Background of Related Art
A data access arrangement (DAA) typically provides a complete telephone line interface to North American and International Public Switched Telephone Networks (PSTNs). A typical modem solution, for instance, comprises a DAA to interface to the telephone line, a modem chipset, and support components such as a host processor.
DAAs vary in complexity. For instance, some DAAs may include ring detect circuitry, a line transformer, and relay functions. Other functions such as a 2- to 4-wire hybrid, Caller ID, handset interrupt, and connect detect may be included in other models of DAAs. International models may also be available which contain special features required by the telecommunications regulations of various countries.
In a digital device such as a modem (e.g., a 56K modem), a DAA is used to provide a barrier between a modem chip set and a telephone line. Included in a conventional DAA for this application are transformers to provide isolation and balance while minimizing signal distortion within communications networks. Located close to the phone jack, the transformers function as an isolation barrier to high voltages which may be present on a telephone line. While minimizing signal distortion, electrical aspects of a DAA and associated telephone line nevertheless inevitably cause some level of frequency response distortion in a transmitted signal, most typically either because of an impedance mismatch With a central office, and/or because of a complex impedance at the central office.
DAA transmit circuitry may follow Norton or Thevenin equivalence rules. However, if Norton equivalency is used (e.g., a current source transmitter as opposed to a voltage source transmitter), the image match circuit causes additional frequency response distortion not present with a Thevenin type (e.g., a voltage source) transmitter.
Maintaining a flat transmit response is important to high-speed applications, particularly in high speed modem applications such as a 56 kbits/s modem. Minimizing transmit frequency response distortion levels is a requirement of such high-speed applications because transmit frequency response distortion tends to slow data rates. Thus, the lower the frequency response distortion, the better the possibility of increasing the data rate of the device.
In particular, when transmitting a signal from a component such as a modem to a telephone line through a DAA, the goal is to have a flat transmit signal at all frequencies. In the United States this is easy to accomplish because the central office termination is typically required to be 600 ohms, which has a flat frequency response. Because of this simple impedance, a pre-distortion filter can be implemented with analog components, e.g., in a Hybrid device. Moreover, analog pre-distortion filters can be used in analog interfaces to compensate for the effects of frequency distortion experienced by a transmitted signal in a country where the central office presents a complex load to a device connected to a telephone line.
Ideally, a pre-distortion filter pre-distorts a transmitted signal before it is transmitted in a way which is opposite to the effects of the frequency distortion which it will experience so that the resulting signal after transmission will match that before transmission.
If the impedance is matched between the modem device and the central office, return loss will be minimized and pre-distortion is relatively simple. Thus, if the DAA of the modem device presents a load (even a complex load) to a telephone line which is identical to the load at the central office, the signal received back from the telephone line will have a flat frequency response (i.e., will not be frequency distorted) with a 6 decibel (dB) loss. This is a minimized return loss.
Unfortunately, digital circuit advances are limited by the conventional analog pre-distortion architectures. One such advance is digital emulation of a complex load of a central office, as devised by the present inventors.
In particular, the inventors have developed an architecture for emulating a complex central office load, entitled IMPEDANCE MATCHING WITH SIGMA DELTA FILTER, filed on Aug. 24, 1999 and having common inventorship with the inventorship of the present application as of its filing, the contents of which are expressly incorporated herein in their entirety.
When emulating a central office load in accordance with the inventors"" impedance emulation concepts disclosed in a previous patent application, all elements of the transmit transfer function fall out but for the central office load. As a result, any transmitted signal will take on the shape of the complex load of the central office. Thus, even if the frequency of the originally transmitted signal is flat, signals passed over the current driven lines will nevertheless be frequency distorted by taking on the shape of the complex load of the central office when transmitted over the telephone line. This can result in a dynamic range of as high as 12 dB.
Thus, complex loads are typically accommodated with an analog pre-distortion filter in a hybrid. However, in digital systems which emulate a load in a DAA, pre-distortion of a transmitted signal would interfere significantly with (or change significantly) the value of the emulated load in the DAA, degrading the benefits of the pre-distortion and making the impedance emulation difficult if at all possible. Moreover, besides the degrading effects caused by a pre-distortion filter on digital processing such as emulation of a complex load, the analog components of a conventional pre-distortion filter and switching mechanisms in a hybrid necessary to accommodate any of a variety of country""s complex loads add to the cost of the DAA.
This equalization must be independent of xe2x80x9cpre-emphasisxe2x80x9d algorithms that exist in a high speed modem transmit signal path such as a V.90 transmit algorithm. The equalization process defined in this disclosure is specifically used to remove the frequency response distortion that occurs with a current source transmitter.
There is thus a need to provide a DAA architecture which allows flattening of the frequency response of a transmit signal by pre-distortion even when emulating a complex load of a central office.
In accordance with the principles of the present invention, an interface for connection to a telephone line comprises a data access arrangement adapted for connection to a central office. A digital pre-distortion filter is in a transmit path in communication with a modem side of the data access arrangement.
A method of flattening a dynamic range of a transmit signal through a data access arrangement in accordance with another aspect of the present invention comprises pre-distorting a transmit signal. The pre-distorted transmit signal is presented to a data access arrangement. The pre-distorted transmit signal is transmitted to a central office together with a signal representing an emulated impedance of a complex load of the central office.